Efforts to reduce the stress associated with dental polymers
used in composite restoratives have focused on new materials as well as use of modified
photocuring protocols. Regarding the latter, accurate characterization of polymerization
rate and limiting conversion is essential for rational interpretation of any
observed differences in stress. Objective: This study compares the effect of conversion,
achieved under a variety of reaction conditions, on polymerization stress.
Methods: Stress specimens (n=3) prepared from a visible
light initiated Bis-GMA/TEGDMA unfilled resin were irradiated at 100 mW/cm2
for either 5, 10, 20, 40, 120 or 240 s or alternatively at 35 mW/cm2
or 240 mW/cm2 for 20 and 40 s, respectively. Dynamic stress was
measured using a cantilever beam tensometer while simultaneous real-time
conversion was monitored using near-infrared spectroscopy. The coupled
conversion and stress data were collected over 15 minutes for each trial.
Results: This analytical approach allows stress evolution
during polymerization to be plotted as a function of conversion so that effects
of reaction rate and final conversion can be differentiated. There is an
obvious nonlinear development of stress with conversion in which stress rises
dramatically beyond approximately 50% conversion. Final conversion values ranged
from 46.0 ± 2.0% (5 s at 100 mW/cm2) to 67.9 ± 0.1% (40 s at 240
mW/cm2), which coincided with the lowest and highest final stress
levels observed (0.11 ± 0.06 and 2.77 ± 0.07 MPa, respectively). Overall, the final
stress increases in a predictable exponential manner (r2= 0.994)
with final conversion and no deviation in the conversion dependence was evident
regarding the polymerization rate.
Conclusions: The technique of simultaneous stress and
conversion evaluation provides critical fundamental information to assist in
understanding polymerization stress development. Under the conditions used
here, final conversion and not reaction rate determines polymer stress.
Support: NIH/NIDCR 2R01DE14227 and a gift from Septodont.
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